The problem can occur due a number of modem settings. The most
frequent is that the echo is not disabled on modems that are to
accept incoming calls. In hayes language, ATE0 disables echo.
If echo is enabled, the login prompt sent by the DTE (computer)
is echoed back to it. It then issues "password:" prompt which
the modem conveniently echoes back again. And the cycle goes on.
Another setting to check is the Q registers which controls the
modem's response to the DTE. The hayes command ATQ0 enables
results codes. The Q register in conjunction with the X and
the V registers, reports back to the DTE the status of the incoming
or outgoing call, and the type of connection established. For
outgoing calls, one would want to enable result codes but not
for incoming calls. If codes are enabled (via ATQ0), when a
call comes in, the modem will send strings (if ATV1 and ATX4 set)
"RINGING", "CONNECTED" etc. to the local DTE. The getty will think
some one is trying to login. It will send password prompt. Trouble
again. Telebit and other intelligent modems has ATQ2 setting
which says "reports codes only when calling out." For others,
you will need to disable it (ATQ1) and enable when calling out.
The DSR setting can be a source of problem. Ideally, modem should
raise DSR only when CD is detected and modem raises CTS. Computer,
such as the VAX, wait 30 seconds after the modem raises DSR. If
CD is not on by that time, the computer will drop DTR causing the
modem to reset. The DSR should be set to be raised only when
carrier has been detected. The hayes setting is AT&S1.
The DCD setting should be such that it reflects the true nature of
the carrier. The CD can be forced on at all times (AT&C0). This
is not good. Set it to AT&C1.
Most modems can reload saved settings from non-volatile memory
on a DTR transition. This is useful for resetting the modem
when a call drops. Use AT&D3 to get this effect.
For dial in, thus, set AT E0 Q1 &C1 &D3 &S1 &W. The &W stores the
settings in memory. They will be reloaded due to &D3.

User Contributions:

Comment about this article, ask questions, or add new information about this topic: